Services Supported by a Modbus Link Master Processor

Communication Functions

Three specific communication functions are defined for sending and receiving data via a Modbus communication channel:

READ_VAR: To read variables
WRITE_VAR: To write variables
DATA_EXCH: To send Modbus requests to another device over the selected protocol

For detailed information on these communication functions, refer to the chapter General Information on M340 Communication Functions.

Data Exchanges

Reading or writing of variables are carried out by addressing following requests to the targeted slave device.

These requests use the READ_VAR, WRITE_VAR, and DATA_EXCH communication functions:

Modbus request	Function code	Communication function
Read bits	16#01 or 16#02	READ_VAR
Read words	16#03 or 16#04	READ_VAR
Write bits	16#0F	WRITE_VAR
Write words	16#10	WRITE_VAR
Other request	all	DATA_EXCH

NOTE: WRITE_VAR can be used in broadcast mode (READ_VAR can't be used in broadcast mode). In this case, the PLC doesn't receive a response. Sending a broadcast request resets the activity bit and the code 16#01 (Exchange stop on timeout) is returned into the second management word.

NOTE: The objects read by Modicon M340 PLC can be of the type and .In this case, READ_VAR function generates a Modbus request: FC 0x2 or 0x4. In a Quantum PLC, it allows accessing the Input Status or Input Status Registers.

More generally, it is possible to send any Modbus requests to a slave device by using the DATA_EXCH communication function.

Cancelling an Exchange

There are two ways of programming that enable an exchange executed by the communication functions to be cancelled:

Using the CANCEL function
Using the communication function cancel bit

For detailed information on cancelling a communication function, refer to EcoStruxure™ Control Expert, Communication, Block Library.

EF is the abbreviation of Elementary Function. This is a block used in a program which performs a predefined logical function. A function does not have any information on the internal state. Several calls to the same function using the same input parameters always return the same output values. You will find information on the graphic form of the function call in the "[functional block (instance)]". Unlike a call to a function block, function calls include only an output which is not named and whose name is identical to that of the function. In FBD, each call is indicated by a unique [number] via the graphic block. This number is managed automatically and cannot be modified. You position and configure these functions in your program in order to execute your application. You can also develop other functions using the SDKC development kit.